Superalloy materials are among the most difficult materials to weld due to their susceptibility to weld solidification cracking and strain age cracking. The term “superalloy” as used herein means a highly corrosion and oxidation resistant alloy with excellent mechanical strength and resistance to creep at high temperatures. Superalloys typically include high nickel or cobalt content. Examples of superalloys include alloys sold under the trademarks and brand names Hastelloy, Inconel alloys (e.g., IN 738, IN 792, IN 939), Rene alloys (e.g., Rene N5, Rene 80, Rene 142, Rene 195), Haynes alloys, Mar M, CM 247, CM 247 LC, C263, 718, X-750, ECY 768, 282, X45, PWA 1483 and CMSX (e.g. CMSX-4) single crystal alloys.
Gas turbine airfoils, both rotating blades and stationary vanes, are often manufactured by casting a superalloy material around a fugitive ceramic core that is then removed to form cooling chambers and channels in the blade. The manufacture of these turbine blades, typically from high strength, superalloy metal materials, involves an initial manufacture of a precision ceramic core to conform to the intricate cooling passages desired inside the turbine blade. A precision die or mold is also created which defines the precise 3-D external surface of the turbine blade including its airfoil, platform, and integral dovetail. A schematic view of such a mold structure 10 is shown in FIG. 1. The ceramic core 11 is assembled inside two die halves which form a space or void therebetween that defines the resulting metal portions of the blade. Wax is injected into the assembled dies to fill the void and surround the ceramic core encapsulated therein. The two die halves are split apart and removed from the molded wax. The molded wax has the precise configuration of the desired blade and is then coated with a ceramic material to form a surrounding ceramic shell 12. Then, the wax is melted and removed from the shell 12 leaving a corresponding void or space 13 between the ceramic shell 12 and the internal ceramic core 11 and tip plenum 14. Molten superalloy metal is then poured into the shell to fill the void therein and again encapsulate the ceramic core 11 and tip plenum 14 contained in the shell 12. The molten metal is cooled and solidifies, and then the external shell 12 and internal core 11 and tip plenum 14 are suitably removed leaving behind the desired metallic turbine blade in which the internal cooling passages are found. In order to provide a pathway for removing ceramic core material via a leaching process, a ball chute 15 and tip pins 16 are provided, which upon leaching form a ball chute and tip holes within the turbine blade that must subsequently be brazed shut.
U.S. Patent Application Publication No. 2010/0200189 (assigned to General Electric Company) discloses a method by which, as shown FIGS. 2A and 2B, the outer end of the airfoil 18 may be closed. In a first step, as shown in FIG. 2A, a tip plate 50 that precisely defines that shape of the cross section of the airfoil 18 is placed on the outer end of the airfoil 18 (without the ceramic casting mold), in contact with the outer wall 19. The tip plate 50 is bonded to the outer wall 19 by laser welding. Laser energy is then directed at the tip plate 50 from the end or the peripheral edges (see arrows “W” in FIG. 2A) so as to produce a through-weld and fuse the outer periphery of the tip plate 50 to the outer wall 19. Next, as shown in FIG. 2B, the tip wall 34 is formed through a freeform laser fabrication process where molten alloy powder is deposited on the tip plate 50 in one or more passes.
In another aspect, U.S. Patent Application Publication No. 2010/0200189 depicts an airfoil 18″ formed by an alternative method. FIG. 3A illustrates the airfoil 18″ in the as-cast condition (without the ceramic casting mold) with an outer wall 19″. The interior of the airfoil 18″ is filled with a suitable metallic alloy powder 68, which is scraped flush or otherwise leveled with the outer end of the outer wall 19″. The powder 68 is sintered together and bonded to the outer wall 19″ by directing laser energy at it, shown schematically at arrow “L” in FIG. 3A. FIG. 3B depicts the airfoil 18″ after the powder 68 has been sintered into a completed tip cap 36″ and the excess powder 68 removed. Once the tip cap 36″ is formed, a tip wall 34″ is formed on top of the tip cap 36″ using a freeform laser fabrication process, as shown in FIG. 3C.
U.S. Patent Application Publication 2015/0034266A1 (assigned to Siemens Energy, Inc.) describes a method of manufacturing a turbine blade where the cavity of the blade is also filled with a support material for subsequent formation of the blade tip. As shown in FIG. 4, method 80 includes step 82 where a superalloy turbine blade is initially cast without a blade tip cap but with tip walls. At step 84, a supporting element is placed in a cavity of the blade. Then at step 86, an additive filler material comprising a metal powder is supported on the supporting element across the opening. Next at step 88, an energy beam is traversed across the additive filler material to melt the material and to thereby form a superalloy cap across the blade tip and is fused to the existing blade tip walls. The method 80 further comprises a step 90 of building a radially extending squealer ridge around the periphery of the cap via additive welding.
In view of the foregoing, and the fact that current blades and vanes tend to be life limited especially at their tips which are very expensive to replace, a need remains for novel methods of manufacturing tips or other components for new-make cast airfoils and field-return repair airfoils. It would be desirable to provide methods that are less time-consuming and more cost-effective. For example, it would be especially beneficial to provide methods that utilize materials that are already existing in airfoil manufacturing facilities and/or methods that find new, secondary uses for such materials, thereby circumventing the need to acquire and waste any new materials.